A probe device comprises a two-channel capacitive measuring sensor together with two further single-channel capacitive measuring sensors so disposed that the tips of the sensors are disposed in a common horizontal plane, the sensors being operable to detect the position and speed of movement of an interface between first and second phases in a system made up of at least two phases, such as a gas-solids fluidized bed. The data obtained when the probe is contacted by a gas bubble in the fluidized bed serve as the basis for them ascertaining the condition of and thus the reaction procedure taking place in the bed.